Symmetry is a big one. The less symmetry in a face, the less attractive a person is. Everyone has some degree of symmetry, and some degree of asymmetry, but the more symmetry, the more attractive you are.

If you combine more than one face, to get a sort of hybrid of the two, the hybrid is likely to be more attractive than either person. And the more you blend together, the more attractive the face becomes. It's almost like the average face is the most attractive one. Average in feature size, distance, etc.

And there are features for the body also. So in a way, you can quantitatively determine how attractive you are. But of course, that only goes so far, as nobody (or almost nobody) cares only about the physical.

Beauty is in the eye of the beholder, and it doesnt necessarily need to be simply exteriors that one pays attention to. Of course one's apperance is a factor, but intelligence can also be eroticised. Would you rather date a vacous tart or a girl with the abilty to converse?

Beauty is already there anyway, and often make-up only makes a woman look ugly. The most beautiful women I know are ones who wear no makeup, and look their most beautiful in the morning in their pj's; sleepy and pretty-eyed.

This is true beyond members of the opposite sex as well. We associate certain places with beauty, or an event, or art, or what have you. What makes these things stand out, what makes them beautiful, is the effect they have on your mind. Beauty is the instant of awe, of the joy that washes over your body whenever you look at/hear/touch/taste/feel said beauty. In that respect, beauty is defined by how we feel, and by our reactions of the world around us. Beauty is exceptional and memorable, ugliness is forgettable and dull. The mind is trained, for it's own good, to disregard ugliness and concentrate on beauty.

Beauty is so desirable because of the way it makes us feel. On a chemical level, I suppose (and I'm no expert here) that beauty triggers the release of endorphins. Emotionally (this I know a little about), beauty inspires feelings of clarity, balance, perception, contentedness, peace, and a general sense of joy.

One more point...Sometimes, when we come across a thing of beauty, we feel like we being clued in or taught something that we don't currently understand, like we seeing a small part of a big picture. Beautiful things often seem to happen spontaneously, and experiencing them makes us feel humble, like we have been blessed with another piece of the puzzle.

Basically, beauty is a very individual quality, not one we can impose external standards on. It is an emotion, indeed, the very font of all emotion.